It is known to install wind turbines onshore or offshore at locations allowing the wind turbine to produce as much energy as possible typically by arranging them in groups. During the recent years, the height and size of the wind turbines have increased together with the maximum power output capable of being produced by these large wind turbines. As the power output is proportional to the size and length of the wind turbine blades, these have also increased in size and length as well as the height of the wind turbine tower. Today, there is an increasing demand for placing such large wind turbines at offshore locations due to noise limitations and other factors.
Offshore wind turbines are normally installed on an offshore foundation (sometimes called near-shore foundation) secured to the seabed either by a monopole driven into the seabed or a gravity or tripod foundation placed on the seabed. Such foundations are, however, expensive and only usable in waters having a depth of about 40 m or less. A floating foundation having one or three buoyancy chambers is normally used for deeper waters; however, such floating foundations are typically very expensive and are often very large and bulky as they have to provide a stable platform for the wind turbine. The thrust of the wind (sometimes called the wind thrust or rotor thrust) acting on the rotor plane depends on the density of the incoming wind and is an important factor when determining the size and weight of such a floating foundation, i.e. the floating element.
Offshore wind turbines are subjected to a wind profile which differs from the wind profile over land. The offshore wind profile has mean wind speeds which are more constant and often higher than an onshore wind profile. The onshore wind profile often suffers from wind shears and turbulences which reduces the mean wind speed. Furthermore, a floating foundation is also subjected to thrusts from marine current and wave movements and the hydrostatic thrust acting on the submerged part of the foundation. The tilting or angular rotation of the wind turbine which causes the wind turbine to oscillate due to the various thrusts acting on the structure becomes a main issue at wind speeds over 10 to 14 m/s, e.g. 12 m/s (also called rated power).
Stall regulated wind turbines are sensitive to the density of the incoming wind which changes during summer and winter and at rated power at a certain wind speed, turbulence will occur on the upwind side of the blade, inducing stalls, and thereby an increasingly amount of loads and vibrations. Such wind turbines are therefore very large, heavy and expensive solutions for offshore use and are therefore not a preferred as solutions. Furthermore, the thrust or load from the wind acting on such a wind turbine influences the rated power situation which may lead to turbulence around the wind turbine blades. This subjects the wind turbine to significant storm loads and stresses, causing the wind turbine to fail. At extreme high wind speeds or wind gusts the loads and stresses experienced by the wind turbine will increase exponentially. This requires the wind turbine to be constructed or rated so that it can withstand such high loads and stresses. In order to achieve these rating requirements defined by IEC, the wind turbine manufacturers typically add reinforcing materials and/or increase the size and strength of various parts of the structure. This in turn increases the production costs of such a wind turbine. Such a wind turbine does not allow the efficiency of the wind turbine to be controlled.
Pitch-regulated wind turbines for offshore use normally pitch the entire wind turbine blade into or out off the wind so that it maintains a relative constant power output in the rated power situation. When the wind turbine blades are pitched out of the wind, the thrust acting on the rotor hub is reduced, and when the wind turbine blades are pitched into the wind the thrust increases. This causes the wind turbine to repetitively tilt or rotate angularly relative to the centre of gravity of the structure which is a major problem when the wind turbine is placed on a floating foundation. As the wind turbine tilts towards the wind it will induce an additional wind thus increasing the relative wind hitting the rotor plane. As the wind turbine tilts backwards away from the wind it will move with the wind thus decreasing the relative wind hitting the rotor plane. This means that if the wind turbine is placed on a floating foundation it will see a repetitive change in the relative wind hitting the wind turbine even if the wind speed of the incoming wind is constant. These negative damping oscillations could have a significant self-enhancing effect on the wind turbine which may overturn the structure or lead to a failure in the structure.
Furthermore, the hydrodynamics and cyclic thrust acting on the floating foundation due to the marine current and waves may also cause the structure to repetitively tilt or rotate angularly relative to the centre of gravity of the structure. This could also have a self-enhancing effect on the structure if the marine natural frequencies substantially match that of the thrust acting on the rotor hub. This oscillation movement of the wind turbine leads to significant stresses and fatigue loads in the structure which requires the wind turbine, in particular the tower, to be reinforced with additional materials so that the structural strength is increased. This in turn increases the production costs of the wind turbine. One way of solving this tilting problem is to increase the size and weight of the floating foundation; however this solution increases the production costs of the floating foundation, thus making it a very expensive solution. This solution may also increase the installation costs due to the increased size and weight of the floating foundation.
WO 03/004869 A1 discloses a wind turbine with three traditional wind turbine blades installed on a floating foundation which is secured to the seabed by a number of anchoring cables. The floating foundation comprises a buoyant chamber formed inside the body of the foundation which allows the foundation to float near the water surface. The buoyant chamber may in one embodiment comprise a number of ballast chambers where the ballast can be adjusted in order to compensate for the tilting of the wind turbine. A weight placed on a movable lever arm at the bottom of the foundation or a tension system, e.g. combined with a pole contacting to the seabed, may be used instead of adjusting the ballast in the ballast chambers. All the disclosed embodiments describe a relatively complex and expensive solution coupled to the floating foundation which increases the size and overall costs of the foundation. Furthermore, the adjustment of the ballast in the different chambers is not considered to provide an acceptable solution, since movement of the ballast between the different chambers takes time and may even start to resonate with the repetitive tilting of the wind turbine.
U.S. Pat. No. 7,156,037 B2 discloses another solution where a wind turbine with three traditional wind turbine blades is installed on an elongated buoyant foundation pivotally secured to the seabed. The foundation comprises a ballast room partly filled with water. The nacelle is coupled to the top of the tower using a tilting joint which tilts the position of the rotor hub so that the rotor axle is always positioned in a horizontal position. An anchoring cable connected to a platform placed on the seabed or three anchoring cables connected to the wind turbine are used to stabilize the wind turbine. This solution has the disadvantage that the tip of the wind turbines may risk hitting the tower when the tower is tilted backwards. This configuration allows for a relatively large tilting movement of the wind turbine which results in relatively large changes in the relative wind speed hitting the rotor plane. The tilting joint furthermore provides a weak point which is likely to fail.
WO 2012/069578 A1 discloses a pitch-regulated wind turbine installed on a floating foundation secured to the seabed by a number of anchoring cables. The foundation comprises one to four buoyant chambers partly filled with ballast and a number of stabilizing arms extending outwards from the foundation. A rotatable propeller is arranged at the bottom of each buoyant chamber and coupled to a control system which controls the operation and direction of the propellers. An accelerometer or a GNSS receiver in form of a GPS receiver is used to measure the oscillation of the wind turbine where the propellers are used to provide a movement in the opposite direction so that the oscillations are dampened. This solution requires the use of a propeller system to provide a relative stable platform for the wind turbine which adds to the complexity and size of the floating foundation, thus increasing the overall costs. Furthermore, this solution requires the use of a number of stabilizing arms or three or more buoyant chambers to compensate for the movement or rotation of the foundation caused by the current and waves.
All these solutions require the use of an additional system to be either coupled to the exterior of the floating foundation or placed inside the floating foundation to counteract the repetitive tilting of the wind turbine.
WO 2005/090781 A1 discloses a floating wind turbine which at the bottom of the wind turbine tower is connected to an anchor located on the seabed. The wind turbine comprises three traditional blades connected to a pitch control system which regulates the pitch angle of the blades according to a mean thrust force acting on the rotor hub for counteracting the movement of the tower. The control system regulates the pitch angle of the wind turbine blades to counteract for the movement of the structure; this places the wind turbine blades in a non-optimal position relative to the wind direction. This decreases the thrust acting on the rotor hub, but also reduces the efficiency of the system. The tower of this three-bladed wind turbine experiences very high loads during a high wind standstill which could lead to a dangerous angular movement of the tower during the standstill.
The thesis “Model-based control of a ballast-stabilized floating wind turbine exposed to wind and waves” of Soeren Christiansen discloses various control methods for controlling a wind turbine placed on a floating foundation. The control methods are based on a 5 MW wind turbine (called Hywind) with three traditional blades placed on a foundation shaped as a spar buoy which is anchored to the seabed using tensioning wires. Paper C of this thesis discloses a control method where the three blades are pitched according to the minimum thrust acting on the rotor hub. However, the structural strength of the main rotor shaft and the drive train need to be increased, since the reduced generator speed causes the generator torque to increase. Paper D discloses another control method utilising a traditional onshore controller for controlling the pitch angle of the blades. This control method requires an additional control loop for stabilising the platform and the entire system. This adds to the complexity and costs of the wind turbine structure.